1. Field of the Invention
The field of the present invention relates to communications and, more specifically, to a multiple user communication system with polarized antennas.
2. Description of the Related Art
In a wireless communication system it is generally necessary for a receiver to distinguish between those signals in its operating region that it should accept and those it should reject. A common method in the art is frequency division (FDMA), in which a separate frequency is assigned to each communication channel. Another common method in the art is time division (TDMA), in which a separate time slot in a periodic time frame is assigned to each communication channel. Yet another method in the art is code division (CDMA), in which a separate spreading code is assigned to each communication channel.
One problem which has arisen in the art is that contiguous coverage of a large area using radio communication has required a cellular configuration with a large number of cells, and thus with only a small number of frequencies available per cell. In an FDMA system, all relatively proximate cells, not just adjacent cells, must operate on different frequencies, and frequencies may be reused only sufficiently far away that stations using those frequencies no longer interfere. For example, in practical FDMA systems, with homogenous conditions and equal-power transmitters, the distance between perimeters of like-frequency cells should usually be at least two to three times the diameter of a single cell. This requirement had led to a seven-cell configuration now in common use for FDMA cellular networks.
While it is desirable to space same frequency cells as far apart as possible in order to minimize interference, the amount of distance that same frequency cells can be separated is limited because the number of frequencies available is limited. This is because the fewer available frequencies, the fewer intermediate cells can be placed between same frequency cells. Different codes have been employed using CDMA techniques in order to reduce the intercell interference in such systems, and thereby reduce the number of frequencies needed and/or the distance of separation between same frequency cells. However, codes may have limited orthagonality (e.g., where a large number of fixed or short length codes are needed), which means that even in the best case they contribute noise to adjacent cells using the same frequency.
Accordingly, it would be advantageous to provide a wireless communication system which allows for a reduced number of required frequencies, or a reduced distance between same frequency cells, or both. It would further be advantageous to provide such a system not requiring the use of CDMA to reduce intercell interference, but which may nevertheless be used in conjunction with CDMA to further limit the number of required frequencies and/or minimize the required distance between same frequency cells.